Well logging pad having a flexible electrode structure

ABSTRACT

In accordance with an illustrative embodiment of the present invention, an improved electrode carrying pad member suitable for engaging the wall of a borehole to investigate the adjacent formations is disclosed. The electrodes carried by the pad member include a plurality of linear, strip-like concentrically arranged and rectangular shaped electrodes. To enable the pad to bend to conform to the curvature of the borehole wall while maintaining good mechanical and electrical characteristics, portions of the electrode segments that are transverse to the longitudinal axis of the borehole are made of flexible spring blades with rigid metal segments attached thereto.

United X 400 a} C:

Espinasse I June 18, 1974 [54] WELL LOGGING PAD A 2,930,969 3/1960 Baker324/10 TR CTURE 3,329,889 7/1967 Tanguy 324/10 X FLEXIBLE ELECTRODE S U3,379,963 4/1968 Saurenman 324/10 [75] Inventor: Roger Espinasse,Essone, France [73] Assignee: Schlumberger Technology Corporatiom NewYork, NY.

[22] Filed: Dec. 15, 1972 211 Appl. No. 315,383

Related US. Application Data [63] Continuation of Ser. No. 134,939.April 19, 1971,

abandoned,

[52] US. Cl. 324/10 [51] Int. Cl G01v 3/18 [58] Field of Search 324/1,10

[56] References Cited UNITED STATES PATENTS 2,712,630 7/1955 Doll 324/102,750,557 6/1956 Bricaud 324/10 Primary ExaminerGerard R. Strecker [57ABSTRACT In accordance with an illustrative embodiment of the presentinvention, an improved electrode carrying pad member suitable forengaging the wall of a borehole to investigate the adjacent formationsis disclosed. The electrodes carried by the pad member include aplurality of linear, strip-like concentrically arranged and rectangularshaped electrodes. To enable the pad to bend to conform to the curvatureof the borehole wall while maintaining good mechanical and electricalcharacteristics, portions of the electrode segments that are transverseto the longitudinal axis of the borehole are made of flexible springblades with rigid metal segments attached thereto.

7 Claims, 4 Drawing Figures PAIENIEB IIW I 3.818.324

- sns'nxnrs v INVENTOR. Roger Espinasse BY a? ATTORNEY PATENTED JUN I81974 SHEET 2 OF 3 1 h wm amm baw QFma -F WELL LOGGING PAD HAVING AFLEXIBLE ELECTRODE STRUCTURE This application is a continuation of Ser.No. 134,939, now abandoned.

This invention relates to tools used in the investigation of geologicalformations through which a borehole passes and, in particular, to thosetools whose active parts operate in the immediate proximity of the wallsof the borehole.

These tools have at least one pad member designed to be movedlongitudinally'near or against the wall of the borehole. This pad ismounted on a linkage arrangement which is in turn supported by a centralsupport member. The central support member is suspended at the end of acable which connects the tools electrically and mechanically to thesurface. This linkage arrangement makes it possible to move the padoutward when desired to apply it against the wall of the borehole or toretract it to a position along the body of the tool.

The pads used for the electrical investigation of formations meet thisgeneral definition. Their active parts consist of a set of electrodeslocated on the front face of the pad which is designed to be appliedagainst the wall of the borehole. Through well-known use of threeelectrodes, a measurement of the electrical characteristics (e.g.,resistivity) of the formation in the immediate proximity of the boreholecan be made.

All sorts of pad structures have already been proposed. Among the firstpads were those which comprised one-piece electrodes, buttons, strips orconcentric circular rings, set in the front face of the pad ininsulating rubber. Examples of such pads can be found in U.S. Pat. Nos.2,712,629 and 2,712,630 granted to H. G. Doll on July 5, 1955. Anexample ofa typical design comprises electrodes having a succession ofbuttons distributed linearly, for example, in the form of a circle or anellipse. These buttons are embedded in a thick plate of insulatingmaterial, such as an elastomer, defining the front face of the pad. Theelectrical connections between the buttons or contacts are set in thismaterial, the pad assembly thus formed being shaped to have a roundedfront face and to conform to the wall, but having only a limitedflexibility. An example of such a pad is found in U.S. Pat. No.2,712,629 granted to M. C. Ferre on July 5, 1955.

Pads provided with a small number of electrodes of large dimensionscovering almost the entire front face are in widespread use. In thesepads, the electrodes are insulated from each other by thin strips, oftenrectangular, of insulating material. An example of such a pad can befound in U.S. Pat. No. 3,132,298 granted to H. G. Doll on May 5, 1964.It has been proposed to provide a degree of flexbility in electrodeslocated on opposite sides of a rigid centralportion of the pad so thatthe electrodes can make better contact with the walls of the borehole inspite of variations in the diameter of the hole. A solution proposed inthis regard consists of making the side wings of these pads with asurface layer of electrically conductive elastomer reinforced by a sheetof flexible conducting material such as perforated sheet steel setbetween the conducting elastomeric material and an insulating elastomer.This solution may be considered for shoes having electrodes of largesurface area and which can thus transmit a sufficientcurrent into theformation even if the conductive elastomer surface layer has arelatively limited conductivity. A pad of this type can be found in U.S.Pat. No. 3,379,963 granted to D. F. Saurenman on Apr. 23, 1968. Othertypes of pads having flexible side members can be found in U.S. Pat.Nos. 3,379,964 and 3,379,965 granted to F. F. Segesman and D. R. Tanguyet 21]., respectively on Apr. 23, 1968. To use this construction forelectrodes of relatively small dimensions may prove to be somewhat lessthan completely desirable because of the inherent resistance aconductive elastomer must have to allow sufficient cohesion andmechanical strength when the pad is rubbed against the borehole wall.

In order to improve the contact between the pad and the wall of theborehole with different types of electrodes, successful use has alsobeen made of hydraulic" pads. Such pads include a bladder-like pad voidof air and partially inflated with a liquid such as oil. One face ofthis pad is secured to a linkage apparatus and the other faceconstitutes the front face in which are mounted electrodes consisting ofseries of metallic buttons or plates of small dimensions. This typeofpad has a very flexible front face which will conform to the roughfeatures of the borehole, laterally as well as longitudinally. It thusmakes it possible to prevent leakage of the current transmitted by theelectrodes directly into the mud, without going through the formation.Thistype of pad is fragile, and the pad front face is worn during therepeated raising and lowering of the tool in boreholes which are notalways in good condition and which are capable of collapsing. Moreover,in formations containing high pressure gases, it sometimes occurs thatthis gas gets into the bladder and causes it to burst when the apparatusis brought to the surface of the earth. Moreover, while this type of padis characterized by its flexibility, it is thick and its lateral partscannot bend beyond a certain limit, thus making its use difficult invery narrow holes.

In general, there are cases where pads of large transverse dimensionsare necessary. It is also very desirable to be able to use the same padfor any size (diameter) borehole. A large pad capable of being usedunder these conditions must have very good flexibility about alongitudinal axis parallel to the borehole axis. It must in fact be ableto conform to the walls of boreholes of large diameter, for which it isin general preshaped, and those having small diameters. Moreover, in thecase of small-diameter holes, a large pad rubs against the wall of thehole even in the retracted position when it is brought to a positionclose to the central support mem ber, for example when lowering the toolthrough the borehole. Such a pad must thus not only be flexible but alsovery rugged to withstand this repeated friction.

The types of pad structures just discussed are, in general, not able tosatisfy all of these requirements. Pads made of solid metal electrodeplates embedded in insulation are in general too rigid to be able toadapt without difficulty to holes of highly variable dimensions. This isparticularly so when the electrodes are of large transverse dimensions.However, such problems are overcome to a considerable degree through useof pads of the type disclosed in the earlier mentioned Saurenman patent.Pads of this type with flexible electrodes of large surface areasforming the lateral or side portions of the structure employ technicalprinciples which are not easily applicable to large pads which have manyelectrodes of limited surface area. With such pads having a number ofsmall cross-section electrodes, it will be very difficult to providegood electrical characteristics, and at the same time, sufiicientmechanical strength, if a conductive elastomer constituted the frontwall engaging face of such small electrodes.

Finally, the difficulties inherent in the use of hydraulic pads in theparticular case have been considered. Their deformation upon contactwith the uneven features of the borehole wall is badly defined andaffects the response of the investigating apparatus in an indeterminatemanner, i.e., the coefficient of proportionality between the actualformation resistivity and the measurement furnished directly by themeasuring appartus may change with deformation of the pad.

It is therefore an object of the invention to provide a reliable padstructure which overcomes one or more of the above-discussed problems.

It is another object to provide an improved pad structure which isdurable under stresses of all kinds encountered while logging boreholesand, which is also flexible enough to mate easily with the walls ofboreholes having very different diameters.

Another object of the invention is to provide a pad capable of beingapplied against the walls of smalldiameter boreholes without requiringexcessively great application forces.

Still another object of the invention is'to provide a pad structurewhose front face has a plurality of concentrically arranged electrodesof relatively large dimensions and which, at the same time, is flexibleenough to conform to boreholes having different borehole diameters.

In accordancewith the present invention, apparatus for use ininvestigating earth formations includes a borehole wall engaging padwhich includes a flexible member made of a relatively flexibleelectrically insulating material, such as neoprene, having a front facesuitable for engaging the wall of a borehole. A plurality of electrodesare embedded in the flexible member. At least a portion of one of theseelectrodes comprises a flexible, relatively thin element which is offsetfrom the front face of the flexible member and a plurality of rigid,highly conductive elements attached to the borehole wall engaging sideof the flexible element. By this arrangement, the pad will be able tobend sufficiently to conform to the curvature of the borehole wall whilestill maintaining good mechanical qualities in spite of the pad rubbingagainst the borehole wall, as will be explained in detail later.Moreover, because of the rigid conductive elements, the electrodes willmake good electrical contact with the formation. In other words, theresistance between the circuits which are coupled with the electrodesand the formation to be investigated will be extremely small.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, thescope of the invention being pointed out in the appended claims.

FIG. 1 is a perspective view of a pad constructed according to theinvention and mounted on a central support member lowered into aborehole;

FIG. 2 is a plan view of such a pad, partially cut by the line II-Il ofFIG. 3;

FIG. 3 is a cross-section view of this same pad taken along the lineIII--III of FIG. 2; and

FIG. 4 is a view similar to that of FIG. 3 of the shoe taken along theline IV-IV of FIG. 2.

Referring now to FIG. 1, a central support member 1 is lowered into aborehole 2 on the end of a suspension cable (not shown) which providesboth mechanical and electrical connection of this support member 1 withsuitable, well-known apparatus located at the surface (not shown).

The cylindrical body of the support member 1 has two longitudinalgrooves 3 into which may fit two pads. In FIG. 1 only one pad 4 isshown. This pad 4 is connected to the support member by articulated arms5 and 6. The arms 5 and 6 are articulated on the body of the supportmember 1 at one of their ends. At the other end, they are articulated ona metallic pad linkage support 8 elongated in the longitudinal direction(parallel to the support member 1). The pad 4 is mounted on the padlinkage support 8 by means of screws which are not visible in FIG. 1.The arms 5 and 6, the rib member 8 and the body of the pad 1 form anarrangement for moving the pad 4 towards and away from the borehole walland which can be activated by suitable motorized means (not shown). Thisdevice may be maintained in the closed position, with the pad 4 thenbeing in the retracted position in the groove 3, or it may be opened sothat the pad 4 will be applied against the wall of the borehole 2 with aselected force.

The front face of the pad 4 shown in FIG. 1 includes a back plate 12made of flexible, electrically insulating material, for example anelastomer such as neoprene.

One of the faces of this plate defines the outer limits of the majorpart of the front face 13 of the pad, i.e., of

the face which is applied against the wall 10 of the borehole. Thisfront face 13 is equipped with a set of five. rectangular electrodes 14to 18 arranged in a concentric (common center point) fashion from thecenter to the periphery. Thus, the four electrodes 15 to 18 are linearor strip-like and are arranged in close rectangular paths around eachother. The electrode 14 is a rectangular plate at the center of the pad.

The front face 13 has-a form which is generally elongated in thelongitudinal direction, i.e., parallel to the axis of the supportmember 1. The lateral edges of the pad are parallel to the longitudinalaxis while the upper and lower edges describe a substantiallytrapezoidal form which narrows down towards the ends 7 and 9 of thispad.

Along its entire length, the pad is reinforced in its central part by aback rib 20 consisting of a support covered laterally with neoprene.This support appears on the back face 19 of the shoe and may be mountedon the pad linkage support 8. The side wings of the plate 12 on eitherside of the back rib 20 are flexible. The back face 19 of these wings iscompletely covered with neoprene.

Referring now to FIGS. 2 and 3 which show the pad 4 separate from thepad linkage support 8 and linkage arms 5 and 6, the support in the backrib 20 of the pad 4, designated 22, can be clearly seen. It is a sort ofparallelepiped box formed by an elongated narrow frame defined by twolongitudinal uprights 23 and 24, two end walls 25 and 26, and a baseplate 28 closing this box on the side of the front face 13. On the backface 19 of the pad, this support 22 is hollow and defines an elongatedcavity 30. The support 22 may be made of a rigid material, such as metalor preferably a hard plastic, or even a more flexible material such asan elastomer. As shown in FIG. 2, it has a series of tapped holes 32 inreinforced portions of the uprights 23 and 24 by means of which it maybe'bolted or screwed to the pad linkage support 8 (FIG. 1). The padlinkage support 8 is rigid, and can thus give the support 22 therigidity which it may be lacking.

Referring to FIG. 2, three pairs of electrical connection pieces 34 aremounted inside the cavity and provided with plugs 35 which may beplugged into suitable sockets for connecting the pad electrically viacon ductors (not shown) to suitable electrical circuits in a fluid tighthousing (not shown) in the support member 1. The connection pieces 34are secured by means of a small bracket 36 secured to the upright 23 ofthis support by means of screws 37. These connecting pieces are alsolinked to the electrodes 14 to 18 by conductors such as 38 and 38.Should the support 22 be molded in an insulating material, plastic. forexample, the necessary insulation for these connecting elements isinherent. If the support 22 is a machined metallic part, an additionalinsulation is provided for the connection pieces 34 and the conductors38. When the support 22 is secured to the pad linkage support 8, thelatter fits in the cavity 30 in which is made the electrical connectionbetween the pad 4 and the support member 1. In order to protect theelectrical connections, the cavity 30 is packed with grease whichprevents the borehole fluids from penetrating to these connections andgives them an excellent service life.

In the present example, still viewing FIG. 2, the two central electrodes14 and 15 are made of rigid metal pieces. The electrode 14 is a platemounted on the base plate 28 of the support 22 by a central screw40-(see FIG. 3 also). The electrode 15 isa rectangular metal stripmounted around the electrode 14 and secured to the base plate 28 (seeFIG. 3) by means of two screws such as 41. These two electrodesdesirably have a thickness of about 3 millimeters. They are separated bya rectangular strip of neoprene 42.

Referring to FIG. 3, the front face of electrodes 14 and 15 appear inthe front face 13 of the pad 4 about l millimeter back from the frontsurfaces of the flexible trode 17 will be described in greater detailwith referv ence to FIGS. 2 and 4.

The two transverse sides 44 and 45 of this frame are essentiallycomposed of hard carbon steel spring blades on which are welded at closeintervals conducting pieces in the form of low carbon steel contacts.These contacts constitute electrode elements, one face of which appearsin the front face l3jof the pad.

In FIG. 4, the transverse side 44 is composed of a spring blade 46 whosecentral section is mounted on the base plate 28 of the support 22. Overthis central section is welded a conducting strip 48 whose length issubstantially equal to the width of the support 22, i.e.,

about 30 millimeters. A screw 49 secures this strip 48' and the springblade 46 to the support 22. A plurality of contacts 50, 51, 52 arelocated on both sides of the support 22 and consist of small rectangularmetal plates welded on the upper face 53 of the spring blade 46 on theside of the front face 13 of the pad.

The contacts 50, 51, 52 have a thickness of about 2 millimeters andtheir front, wall engaging faces'are not covered with the neoprene ofthe plate 12. These faces, as well as those of the neoprene strips whichseparate them (FIG. 4) are set about 1 millimeter back from the frontface of this plate for reasons well known in the well logging art. Theconductive drilling mud will fill these recesses thus enabling theelectrodes to make electrical contact with the borehole wall and theformations therebehind.

In the present example, the spring blade 46 has a thickness ofthree-tenths of a millimeter and a width of about 5 millimeters, therebygiving it a very high flexibility perpendicular to its plane, withoutany danger of permanent set. The choice of the thickness and the widthof these blades is very important to obtain a pad having the desiredproperties. A relatively small increase in the dimensions of the springblades quickly leads to insufficient performance as regards theflexibility of the pad around a longitudinal axis.

The transverse dimension of the contacts 50, 51 and 52 and their spacingare also factors influencing the deflection which an electrode-carryingframe of given width may attain upon deformation. By way of example, thetransverse dimension of the contacts 51 and 52 is about 5 millimetersand that of the contact 50 about 6 millimeters. Only the portions of thespring blade 46 which are situated between the contacts play a part inthe transverse flexibility of the frame. This is one reason why, if onewishes to minimize the force used for applying the pad against the wall,in particular in the case of small-diameter holes, it is very importantto have blades of small thickness and small width. In the pad presentlydescribed, the values of the spacing between contacts are of the sameorder of magnitude as the transverse dimension of these contacts. Thesespacing values are relatively low because one endeavors to haveelectrodes with the highest possible continuity from the electricalstandpoint. This is particularly important for electrodes which emitcurrent into the formation.

The two parallel spring blades 46 secured to the support 22 on eitherside of its transverse axis of symmetry (line III-III in FIG. 2) areperpendicular to the longitudinal axis of the pad. On each side of thesupport 22, their ends are connected by a rigid connecting strip 60,shown in plan in FIG. 2 and in section in FIGS. 3 and 4. This strip of 2millimeter thickness, like the contacts, is set in the neoprene plate12, and only its front face appears slightly set back from the frontsurface of this plate. These strips are welded onto the ends of the twospring blades 46 and they close the frame associated with the electrode17. This frame is thus rigid in the longitudinal direction through thecombined action of the two rigid strips 60 and the support 22 and has atransverse flexibility because of the spring blades 46. Owing to thesespring blades 46, there is not only a mechanical continuity but also anelectrical continuity between the parts of this frame. Its transverseside 45 is connected electrically to the connection piece 34' by theconductor 38 shown in FIG. 2. The front surfaces of the contacts 50, 51and 52 and of the strips 60 constitute the active faces of the electrodeproper, distributed over the front face 13 along a closed linear orstrip-like path.

The frames associated with the electrodes 16 and 18 are formed in amanner similar to frame 17.. Thus, the frame 16 has two transversespring blades 62 mounted on the support 22 on either side of thetransverse sides of the electrode 15. The two spring blades 62 areconnected by two longitudinal strips 63 parallel to the axis of the pad.The frame also has two conducting strips 64 similar to the strips 48secured to the support 22. It will be noted in FIG. 2 that spring blades62 are extended from the support 22 beyond the strip 63, with their ends62 providing an additional anchoring of the electrode frame 16 in theflexible neoprene plate 12.

Likewise, the external electrode frame 18 has two transverse sides 66and two longitudinal sides 72 (see FIG. 2). The two transverse sidesinclude spring blades similar to the blades 46 whose central portionsare secured to the support 22. In FIG. 2, the solid central portionsofelectrode frame 18 are designated 76. A succession of contacts 67 to 70are located on both sides of the central strips 76. The ends of theseblades are connected on both sides to two strips 72 whose ends 73 areslightly rounded and which define the longitudinal edges of the pad.FIG. 3 shows that these strips are slightly thicker than the strips 60and 63, by about 4 millimeters, and that their external face 74 definesthe lateral or transverse limit of the flexible neoprene plate 12. Bythis construction, the side of the flexible plate 12 is protectedagainst tearing.

As shown clearly in FIG. 2, the frames associated with the electrodesare mounted concentrically around each other on a surface substantiallyparallel to the front face of the shoe. The side sections (60, 72, 63,etc.) of these electrode frames are parallel to the longitudinal axisofthe pad and the end sections (66, 44, 64, etc.) are all perpendicularto this longitudinal axis. The sides and ends of the electrode aresymmetrical relative to the longitudinal axis of the pad (which passesthrough the point 40 in FIG. 2) and the transverse axis of the pad(which also passes through point 40) respectively such that all of therectangular electrodes have a common center point. The neoprene. plate12 is shaped so that the pad face 13 is slightly convex and the neopreneholds the frames in the desired position. The longitudinal parts ofthese frames are rigid thereby requiring the pad to bend cylindricallyabout a longitudinal axis of the pad upon pressure being applied.

The electrode frames mounted on the support 22 give a fragile appearanceowing to their dimensions, particularly for the large frames such as 17and 18. The elastomer plate 12 in which they are held along with thesupport 22 provides cohesion and structural integrity of these partswhile also insulating the electrodes from each other. It gives theassembly a resistance to forces tangent to the front face 13. Thisresistance is reinforced by the blades in the flexible plate 12. On ei--ther side of the back rib 20, the thickness of the plate is reduced to 8millimeters thereby allowing it to bend in response to a limited forcewhich is compatible with the wearability of the front surface of theshoe.

The flexible plate 12 covers all the spring blades and the side faces ofthe different contacts and strips, with the exception of the externallateral faces 74 of the strips 72. It also covers the external walls ofthe longitudinal uprights 23 and 24 of the support 22 as shown in FIG. 2and 3. It adheres to all these parts because of a suitable treatmentduring molding.

It is noteworthy that, in spite of their thinness, the spring bladeswhich support the transverse sides of the flexible frames do not cut theelastomer during temperature changes owing to the presence, at closeintervals, of contacts which act as reinforcement along these blades andprevent the internal shearing by the plate under the effect of thedifferent expansions of the steel and the neoprene.

To manufacture a pad of the type just described, the following proceduremay be used. On an elongated hollow support 22, suitably molded andmachined, the electrode frames 14 and 18 are secured by screws. Theframes are electrically connected by means of conductors 38 to theelectrical connection pieces 34 which have been previously mounted onthis support. The assembly thus formed is placed in a mold whose bottomportion has the form of the front face 13 (see FIG. 3), after all theparts to be held in the elastomer are covered with adhesive. The frontface of the contacts and the conducting strips are not covered withadhesive and fit into suitable recesses in the mold. The depth of theserecesses corresponds to the inset of the faces of the electrodes in thefront face of the pad. An elastomer is injected into the mold on theback face of the pad according to the technique known as transfermolding. In this technique, the softened elastomer penetrates into themold through a multiplicity of openings made in a plate closing off themold on the back side of the pad. Under the action of the elastomerinjection pressure, the electrode frames are thrust to the bottom of themold against the corresponding recesses.

As regards the frames proper, they are made by using individual steelblades and contacts having substantially the same composition and placedon a welding fixture to be fusion welded in argon. After this operation,the frames are removed from the fixture and undergo a heat treatmentwith tempering intended to give the blades their optimum elasticproperties.

By way of example, a pad constructed as described above has a width ofI36 millimeters between the edges 74 of the strips 72 (see FIG. 3) and alength (along the pads longitudinal axis) of 300 millimeters. The backrib 20 has a width of 40 millimeters and the greatest thickness of thepad is about 20 millimeters. The large electrode 18 is millimeters longand has the same width as the pad 4. The thickness of the active pieces(those pieces which come in direct contact with drilling fluid) of theelectrodes is about 5 millimeters.

This pad may be easily bent by hand and, when bent, takes on acylindrical profile. By way of example, with the dimensions indicated inthe description above, the application force of such a pad against thewalls of a hole of about 15 centimeter diameter is 8 kilograms. Abouthalf of this force is balanced by the spring blades, the other halfbeing balanced by the inherent elasticity of the neoprene. Tests haveshown that this pad was very rugged during its use in boreholes; thatthe elastomer does not tear; that the conducting electrode contactsremain securely in place in the front face because of their attachmentto the spring blades set in the elastomer; and that they are protectedagainst corrosion and wear.

The pad described above constitutes an example of an embodiment of theinvention which has actually been'developed. The pad so described isparticularly useful with the investigating system described in copendingapplication Ser. No. 815,265 filed by N. A.

Schuster on Apr. 7, 1969. It is however clear that many variations arepossible without departing from the basic principles or the scope of theinvention. It is particularly evident that these principles are capableof being applied to other types of pads with different arrangements,forms and number of electrodes. Similar arrangements may be adopted formaking pads with circular or elliptical electrodes.

While there have been described what are at present considered to bepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,intended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. In a borehole wall engaging apparatus for use in investigating earthformations surrounding a borehole, a combination comprising a boreholewall engaging pad having a relatively flexible member made of arelatively flexible, electrically insulating material and having a frontface suitable for engaging a borehole wall, a plurality of electrodesembedded in said member in proximity of the front fact thereof to makeelectrical contact with the borehole wall, at least one portion of atleast one electrode comprised of a flexible relatively thin strip-likeelement which is offset from the front face of said flexible member, andat least one rigidly highly conductive element attached to said flexiblestrip-like element and extending outwardly toward the front face of saidpad, whereby said wall-engaging member with electrode embedded thereinwill be able to bend to conform to the borehole wall curvature.

2. The apparatus of claim 1 further including a rigid support membersecured to the back, non-borehole wall engaging side of said flexiblemember, a central support member adapted for movement through theborehole and linkage means coupled between said rigid support member ofsaid flexible member and said central support means for coupling saidflexible member to said central support member.

3. In a borehole wall-engaging apparatus for use in investigating earthformations surrounding a borehole, the combination comprising: aborehole wall engaging pad having a relatively flexible member made ofarelatively flexible electrically insulating material and having a frontface suitable for engaging a borehole wall,

a plurality of electrodes embedded in said member in the proximity ofthe front face thereof to make electrical contact with the boreholewall, said electrodes comprising a central relatively rigid electrode,and at least one electrode surrounding said central electrode, saidsurrounding electrode having a first portion comprising a relativelyrigid linear strip-like element aligned along the front face of theflexible member parallel to a given axis, and a second portion alignedsubstantially perpendicular to said axis, said second portion comprisinga relatively flexible strip-like element having at least one rigidhighly conductive element attached to said flexible strip-like elementand extending toward the front face of said pad, whereby said pad canbend around an axis parallel to said given axis.

4. The apparatus of claim 3 wherein said plurality of electrodescomprises a plurality of electrodes surrounding said central electrode,said plurality of surrounding electrodes being concentrically arrangedaround said central electrode, each of said surrounding electrodescomprising said first portions aligned parallel to said axis and saidsecond portions substantially perpendicular to said axis.

5. The apparatus of claim 4 wherein said flexible member has a rigidsection aligned parallel to said given axis and wherein the secondportion of said electrodes surrounding said central electrode furthercomprise a relatively rigid-element located in the relatively rigidsection of said flexible member and wherein said flexible strip-likeelements are located in the relatively flexible portions of saidflexible member.

6. In a borehole wall-engaging apparatus for use in investigating earthformations surrounding a borehole, the combination comprising; aborehole wall-engaging pad which includes a relatively flexible membermade of relatively flexible, electrically insulating material and havinga front face suitable for engaging a borehole wall, a longitudinallyextending rigid support member secured to the backside of said flexiblemember and defining a given area on said backside, thus causing acorresponding portion of said flexible member to be relatively'rigid, aplurality of electrodes embedded in said flexible member in theproximity of the front face thereof to make electrical contact with theborehole wall, said electrodes comprising a relatively rigid centralelectrode located in the relatively rigid portion of said flexiblesupport and a plurality of electrodes surrounding said central electrodeand having relatively rigid portions parallel to said longitudinallyextending rigid support member, those portions of said surroundingelectrodes which are transverse to said longitudinally extending supportmember having rigid sections in the relatively rigid portion of saidflexible support and relatively flexible sections having at lease onerigid highly conductive element attached to said flexible section andextending toward the front face of said pad member in the non-rigidportion of said flexible support.

7. The apparatus of claim 6 wherein the flexible sections of theelectrode comprise a strip of flexible, spring-like conductive materialhaving a front face which faces the front face of said flexible member,said at least one rigid, highly conductive element secured to the frontface of said strip of flexible metal whereby said pad will beable toconform to the borehole wall curvature.

1. In a borehole wall engaging apparatus for use in investigating earthformations surrounding a borehole, a combination comprising a boreholewall engaging pad having a relatively flexible member made of arelatively flexible, electrically insulating material and having a frontface suitable for engaging a borehole wall, a plurality of electrodesembedded in said member in proximity of the front fact thereof to makeelectrical contact with the borehole wall, at least one portion of atleast one electrode comprised of a flexible relatively thin strip-likeelement which is offset from the front face of said flexible member, andat least one rigidly highly conductive element attached to said flexiblestrip-like element and extending outwardly toward the front face of saidpad, whereby said wall-engaging member with electrode embedded thereinwill be able to bend to conform to the borehole wall curvature.
 2. Theapparatus of claim 1 further including a rigid support member secured tothe back, non-borehole wall engaging side of said flexible member, acentral support member adapted for movement through the borehole andlinkage means coupled between said rigid support member of said flexiblemember and said central support means for coupling said flexible memberto said central support member.
 3. In a borehole wall-engaging apparatusfor use in investigating earth formations surrounding a borehole, thecombination comprising: a borehole wall engaging pad having a relativelyflexible member made of a relatively flexible electrically insulatingmaterial and having a front face suitable for engaging a borehole wall,a plurality of electrodes embedded in said member in the proximity ofthe front face thereof to make electrical contact with the boreholewall, said electrodes comprising a central relatively rigid electrode,and at least one electrode surrounding said central electrode, saidsurrounding electrode having a first portion comprising a relativelyrigid linear strip-like element aligned along the front face of theflexible member parallel to a given axis, and a second portion alignedsubstantially perpendicular to said axis, said second portion comprisinga relatively flexible strip-like element having at least one rigidhighly conductive element attached to said flexible strip-like elementand extending toward the front face of said pad, whereby said pad canbend around an axis parallel to said given axis.
 4. The apparatus ofclaim 3 wherein said plurality of electrodes comprises a plurality ofelectrodes surrounding said central electrode, said plurality ofsurrounding electrodes being concentrically arranged around said centralelectrode, each of said surrounding electrodes comprising said firstportions aligned parallel to said axis and said second portionssubstantially perpendicular to said axis.
 5. The apparatus of claim 4wherein said flexible member has a rigid section aligned parallel tosaid given axis and wherein the second portion of said electrodessurrounding said central electrode further comprise a relativelyrigid-element located in the relatively rigid section of said flexiblemember and wherein said flexible strip-like elements are located in therelatively flexible portions of said flexible member.
 6. In a boreholewall-engaging apparatus for use in investigating earth formationssurrounding a borehole, the combination comprising; a boreholewall-engaging pad which includes a relatively flexible member made ofrelatively flexible, electrically insulating material and having a frontface suitable for engaging a borehole wall, a longitudinally extendingrigid support member secured to the backside of said flexible member anddefining a given area on said backside, thus causing a correspondingportion of said flexible member to be relatively rigid, a plurality ofelectrodes embedded in said flexible member in the proximity of thefront face thereof to make electrical contact with the borehole wall,said electrodes comprising a relatively rigid central electrode locatedin the relatively rigid portion of said flexible support and a pluralityof electrodes surrounding said central electrode and having relativelyrigid portions parallel to said longitudinally extending rigid supportmember, those portions of said surrounding electrodes which aretransverse to said longitudinally extending support member having rigidsections in the relatively rigid portion of said flexible support andrelatively flexible sections having at lease one rigid highly conductiveelement attached to said flexible section and extending toward the frontface of said pad member in the non-rigid portion of said flexiblesupport.
 7. The apparatus of claim 6 wherein the flexible sections ofthe electrode comprise a strip of flexible, spring-like conductivematerial having a front face which faces the front face of said flexiblemember, said at least one rigid, highly conductive element secured tothe front face of said strip of flexible metal whereby said pad will beable to conform to the borehole wall curvature.